In geometry, the subtangent and related terms are certain line segments defined using the line tangent to a curve at a given point and the coordinate axes. The terms are somewhat archaic today but were in common use until the early part of the 20th century. ==Definitions== Let P = (x, y) be a point on a given curve with A = (x, 0) its projection o...
• (n.) The part of the axis contained between the ordinate and tangent drawn to the same point in a curve.
Sub·tan'gent noun (Geom.) The part of the axis contained between the ordinate and tangent drawn to the same point in a curve. 
A subtangent is the segment of a produced axis intercepted between an ordinate and a tangent, both drawn from the same point in the curve.
the part of the x-axis cut off between the ordinate of a given point of a curve and the tangent at that point.